cmd/compile/internal/syntax: improved scanner tests

This is one of several changes that were part of a larger rewrite
which I made in early 2019 after switching to the new number literal
syntax implementation. The purpose of the rewrite was to simplify
reading of source code (Unicode character by character) and speed up
the scanner but was never submitted for review due to other priorities.

Part 1 of 3:

This change contains improvements to the scanner tests.

Change-Id: Iecfcaef00fdeb690b0db786edbd52e828417141b
Reviewed-on: https://go-review.googlesource.com/c/go/+/221601


Reviewed-by: Matthew Dempsky <mdempsky@google.com>